1. Field of the Invention
This invention relates to thin film semiconductor devices fabricated on insulative substances, such as thin film transistors, three-dimensional LSI devices, etc., which are applied to active matrix liquid crystal displays, etc., to a process for fabricating the devices, and silicon films. Specifically this invention relates to a process for fabricating a thin film semiconductor device by low-temperature processing the steps of which are conducted at a maximum temperature equal to or lower than about 600.degree. C.
2. Related Background Art
Recently, accompanying the enlargement of screens of liquid crystal displays, and increases of their resolutions, the driving method for liquid crystal displays has changed from the simple matrix method to the active matrix method, and increasingly larger volumes of information can be displayed. The active matrix method enables a liquid crystal display to have more than hundred thousands of picture elements, and has one switching transistor for each picture element. As substrates for such liquid crystal displays, transparent insulative substrates, such as fused quartz plates, glass or others, are used, which enable transparent-type displays to be obtained.
But in order to advance the enlargement of the display screen and lower its price it is essential to use inexpensive ordinary glass as an insulative substrate. In these circumstances, a process for fabricating thin film transistors for operating an active matrix liquid crystal display on an inexpensive glass substrate with high performance, retaining this economy has been required.
As the channel semiconductor layer of a thin film transistor, usually amorphous silicon or polycrystalline silicon is used, but polycrystalline silicon, which has higher operational speed, is more advantageous for the case that a thin film transistor is integrated up to the driving circuit.
Conventionally in fabricating such thin film transistor, thermal oxidation has been used to form a gate insulating layer. That is, to form the gate insulating layer after the formation of a channel silicon layer, a substrate is inserted into an oxidizing ambient atmosphere containing oxygen, (O.sub.2), laughing gas (N.sub.2 O), vapor (H.sub.2 O), etc. to raise its temperature to 800.degree.-about 1100.degree. C. and partially oxidize the channel silicon layer. On the other hand, various processes have been tried in fabricating thin film semiconductor devices using polycrystalline silicon at maximum processing temperatures below about 600.degree. C. at which inexpensive ordinary glass can be used. They are exemplified by the process in which a channel semiconductor layer is formed as-deposited polycrystalline silicon which is prepared by low pressure chemical vapor deposition (LPCVD) with the deposition temperature of about 600.degree. C. or more, and then a gate insulating film is formed by electronic cyclotron resonance plasma CVD (ECR-PECVD) and is further subjected to hydrogenation by e.g., hydrogen plasma radiation. They are also exemplified by the process in which an amorphous silicon thin film is deposited on a channel semiconductor layer, then is heat-treated for about 24 hours at 600.degree. C., and then a gate insulating film is formed by atmospheric pressure chemical vapor deposition (APCVD) and is subjected to hydrogen treatment (Japanese J. Appl. Phys. 30L 84, uL91).
But a number of problems have been pointed out with the above-described prior art processes. Firstly, a problem with the formation of a SiO.sub.2 film by thermal oxidation is the heat resistance of thin film layers and a substrate below the oxide film because the formation of the oxide film involves heat treatment at a high temperature above 800.degree. C. For example, in the fabrication of switching transistors for a large screen liquid crystal display, nothing other than very expensive fused quartz can stand the high temperatures. In three-dimensional LSI devices as well, this thermal oxidation cannot be practically used because the lower layer transistors are deteriorated by the high temperatures.
Next, a problem with the process in which a channel semiconductor layer is formed by LPCVD as-deposited poly-crystalline silicon, and a gate insulating film is formed by ECR-PECVD and is further subjected to hydrogen plasma treatment is that the resultant thin film semiconductor has a mobility as low as 4-5 cm.sup.2 /v.multidot.sec., which is so far insufficient for thin film semiconductor devices. In addition, this hydrogenation treatment, which is conducted for the improvement of properties of the thin film semiconductor device, etches parts of the various thin films thereof, with the adverse result that some of a number of the thin film semiconductor devices are broken. A problem with the process in which an amorphous silicon thin film is deposited as a channel semiconductor layer and is subjected to heat treatment at about 600.degree. C., and then a gate insulating film is formed by APCVD and is subjected to hydrogenation treatment by hydrogen plasma radiation or others is that the resultant thin film semiconductor device has an interface trap level as high as about 10.sup.12, and exhibits properties of the depletion-type semiconductor device, which is so far insufficient for the thin film semiconductor device. In addition, the same problem as that involved in the hydrogenation treatment in the previous process is still unsolved, with the adverse result that thin film semiconductor devices cannot be fabricated homogeneously and stably on a large area.
In these circumstances has been expected a thin film semiconductor device which has a high mobility and, on the other hand, a clean MOS interface and a low interface trap level, and does not exhibit depletion, and a process which can fabricate such thin film semiconductor devices having these advantages homogeneously and stably on a large area and is free from the hydrogenation treatment in the fabricating steps of such thin film semiconductor devices.